This invention relates to the technical field of a method of compressing/extending color reproducing space, a color reproducing method and a color reproducing apparatus.
More particularly, the present invention relates to a method of performing transformation for allowing color reproducing spaces to correspond with each other when shapes and sizes of the respective color reproducing spaces of image input/output devices are different from each other which is capable of corresponding with an image input/output device having a different color reproducing space such that the initial color gamut is maintained in a smooth manner and the initial color appearance and gradation are preserved and is, moreover, capable of adjusting such correspondence as desired. More specifically, the present invention relates to a color reproducing method and a color reproducing apparatus which are adapted to be utilized in at least one of the cases in which print image data is outputted externally as monitor image data or the similar image data and the thus externally outputted image data is transformed back to the print image data which is to be outputted.
Heretofore, a color CRT monitor, a color liquid crystal monitor and the like and a color printer and the like have been popularized and each has widely been used as an image input/output device (hereinafter referred to simply as “image I/O device”) of a color image.
Such image I/O devices can either represent a color image having a desired color by controlling image data ordinarily related to R (red), G (green) and B (blue) or C (cyan), M (magenta) and Y (yellow) or output the color image as a print. However, since color image data as described above are dependent on output characteristics, spectral sensitivity characteristics of the image I/O device, when the color image data is outputted to the image I/O device having different characteristics, it is necessary to perform color transformation of image data taking such characteristics into consideration. Particularly, since spectral sensitivity characteristics of the color CRT monitor or the color liquid crystal monitor and those of the color printer are substantially different from each other, it is necessary to optimally perform color transformation to allow color appearances to coincide with each other such that, for example, the color of the image represented on the color CRT monitor coincides to some extent with the color of the image outputted to the color printer.
In order to perform such color transformation, it is necessary to allow a color reproducing space which can be represented on the image I/O device to correspond to the color reproducing space of the image I/O device to which the initial data is to be transformed; namely, the color transformation which can construct a one-to-one mapping between color points constituting an inside area of the color reproducing space represented on the image I/O device and color points in the color reproducing space of the image I/O device to which the initial data is to be transformed; that is, a method of compressing or extending the color reproducing space is necessitated. Moreover, in such a case, it is necessary to compress or extend the color reproducing space such that not only hue, but also lightness and chroma are maintained in a color gamut in a smooth manner and also the initial color appearance or gradation is preserved, even after the color transformation.
As the color reproducing space as described above, the color reproducing space on a uniform color space to be formed by the image data which does not ordinarily depend on the image I/O device, namely, such as image data of psychometric lightness L* (lightness) and perceived perceived psychometric chromaticities a* b* (hue and chroma) in a CIEL*a*b* color matching system obtained by using tristimulus values (X, Y, Z) of a CIEXYZ colormatching system specified by the International Commission on Illumination (Commission Internationale de l'Étclairage —CIE) or otherwise has been utilized.
By utilizing the color reproducing space on the uniform color space, since, for example, characteristics of color reproducing spaces of these apparatuses are both linear and, therefore, features of color reproducing spaces of these apparatuses are close to each other between the color CRT monitor or the color liquid crystal monitor and the color scanner, correspondence (color matching) between points within the color reproducing spaces can easily be established by compressing or extending the color reproducing space. However, in an additive color mixing system which performs image representation using a transmitted light of apparatuses, such as the color CRT monitor, the color liquid crystal monitor and the like, the color gamut is wide even in a high lightness region thereby outputting a bright color, whereas, in a subtractive color mixing system which performs image representation using a reflected light of apparatuses, such as a color printer and the like, a color with a high chroma can not adequately be outputted in a high chroma range and, moreover, the color gamut is wide rather in a relatively low lightness region so that the color reproducing space is substantially different from that of the additive color mixing system; hence it is difficult to establish a correspondence between the color CRT or the color liquid crystal monitor and the color printer by compressing or extending the color reproducing space.
Moreover, since respective edge portions of the color reproducing spaces of two color printers are each in a nonlinear, roundish form, it is difficult to establish a correspondence between such color printers by compressing or extending the color reproducing space.
With respect to compression or extension of the color reproducing space as described above, Japanese Patent Publication No. 2845523 proposes a simplified method in which the color gamut is extended in accordance with a ratio of color gamuts and, moreover, when the color reproducing space is subjected to extension mapping in a direction of chroma, a mapping transformation is performed only on a peripheral portion and not performed on the central portion in which color gamuts are overlapped. As another case, Unexamined Published Japanese Patent Application (kokai) No. 5-298437 proposes an image processing apparatus in which only chroma is compressed while hue and lightness on the color reproducing space are fixed. However, none of the above-cited Japanese Patent publication and application establishes a correspondence between the color reproducing spaces taking into full consideration the form of color reproducing space to which the initial data is to be transformed or performs processing of adjusting the correspondence between color reproducing spaces in accordance with customer's preference; thus none of them can establish the correspondence between the color reproducing spaces to an extent with which the customer is satisfied such that the color gamut is maintained in a smooth manner and the initial color appearance and gradation are preserved.
In still another case, Unexamined Published Japanese Patent Application (kokai) No. 7-12283 proposes to establish a correspondence to a different color reproducing space by constructing a model of the color reproducing space using a finite element method, inputting an elastic modulus to generate an elastic deformation. However, it is necessary to input the elastic modulus for each minute area of the finite element and, moreover, to adjust/specify the elastic modulus by the customer; therefore, it is troublesome to perform such adjusting/specifying work and also it is difficult to establish the correspondence to a transformed color.
On the other hand, as the digital technology has advanced, a multi-media which performs transmittance of color image information among a multiple of media, for example, an image on a hardcopy, such as a picture, a printed matter, a copy or the like, an image on a softcopy, such as a television, a monitor or the like, have been developed. In such a multi-media system, color image information processing has been performed by means of a digital color image signal. For example, for the purpose of digital service or the like of the picture, a print image data is in some cases transformed into a monitor image data or the similar data which is subsequently to be outputted. In this case, when a reproduction print is performed using the same image data, the same image is required to be reproduced, namely, a coincidence between the images is required.
However, various image I/O devices as described above have each an individual color space, namely, a reproducible color gamut. Therefore, in order to correctly reproduce by another image I/O device color image data, for example, R (red), G (green), B (blue), C (cyan), M (magenta), Y (yellow) and the like inputted by an image I/O device, it is necessary that color spaces of respective image I/O devices are transformed such that they correspond to each other appropriately, for example, in an appropriate proportion.
Ordinarily, the color gamut of the printer is narrower than that of the display so that, if the customer obtains a satisfactory image after having performed a color adjustment looking at the image represented on the display and then the image data thereof is outputted to the printer without being subjected to any change, an intended output image can not be obtained in some cases.
Moreover, it is necessary to perform a sufficient reproduction also for image data having a color gamut which is wider than that of a picture, such as a CG (computer graphic) image or the like.
Heretofore, various methods have been studied in order to optimally perform such color reproductions. For example, there exists a method which changes color reproduction tables in accordance with image sources. Another method is disclosed in Unexamined Published Japanese Patent Application (kokai) No. 9-135360 in which a plurality of transformations are performed on image data and then results of such plurality of transformations are outputted after having been mixed in accordance with the customer's preference. Moreover, U.S. Pat. No. 5,734,802 discloses a method which has both pictorial and CG image look-up tables, synthesizes these look-up tables after they are weighted in accordance with the color gamut of an input signal to produce a mixed look-up table whereupon input image data having both pictorial and CG image factors are appropriately transformed into output image data.
However, in the above-described conventional color reproducing methods, there exists a problem of coincidence of images when print image data is transformed to monitor image data or the similar image data, then outputted externally and the thus externally outputted image data are transformed back to the print image data, or an inappropriate reproducibility when data of, for example, a CG, a digital camera or the like are processed to be externally outputtable data and then outputted as a print.
For example, in the above-described Unexamined Published Japanese Patent Application (kokai) No. 9-135360, images with different mixing ratios are produced for each customer as a result of a plurality of transformations as described above so that, when these data are returned to a lab shop or the like, there is one problem that these data can not be used. Moreover, in the above-described U.S. Pat. No. 5,734,802, there exists another problem that no consideration has been paid to reversibility from the print image data to the monitor image data.